Smart contracts are computer protocols that facilitate, verify, or enforce the negotiation or performance of a contract, or that make a contractual clause unnecessary. Smart contracts often emulate the logic of contractual clauses. Proponents of smart contracts claim that many kinds of contractual clauses may thus be made partially or fully self-executing, self-enforcing, or both. Smart contracts aim to provide security superior to traditional contract law and to reduce other transaction costs associated with contracting.
Computing platforms and software that can be organised for rational decision-making methods which achieve meaningful execution are known as agoric systems. These type of contracts have been traditionally drafted by lawyers.
The real-world implementation of a smart contract that gained mainstream coverage was The DAO, a distributed autonomous organization for venture capital funding, which was launched with US$150 million in crowdfunding in May 2016 and was hacked and drained of approximately US$50 million in cryptocurrency three weeks later.

Source: WikiPedia